Number bonds to 20

My ds was given a learning objective to learn his number bonds to 20. We've been doing this and he's been doing well. My other child asked why they are important and I wasn't sure. Can anyone tell me please?

if he knows number bonds to 10 (8+2, 7+3, etc), then make sure he is noticing that they are used to help with learning the bonds to 20 (18+2, 17+3, etc is really one ten, plus the same number bond in the units) rather than just thinking of them as another lot of facts to memorise. (Some children don't automatically notice the pattern of the teen numbers, partly because they sound different when you say them, and haven't really got a good grasp of the tens and units system yet). If he can use the tens bonds to help with bonds to 20, then he will start to realise the usefulness (because it can be applied, for example, to bonds to 30: 28+2, 27+3, in the same way; and then later, in group of 10, so 70+30, 80+20, and then on to more complicated questions, as described above). They are also useful for subtraction (e.g., knowing 7 and 3 make 10 leads to understanding that if you take 7 away from the 10, you're left with the 3, and vice versa. Then you can apply it to 20 take away 7). Physical apparatus such as cuisinaire rods, lego pieces, abacus, numicon, Smarties, mix of 10p and 1p coins, etc is really good for seeing it visually.